1. Field of the Invention
The present invention relates to a cellulose acylate film that is useful for application in silver halide photographic light-sensitive materials, or in a polarizing plate, a polarizing plate protective film, a retardation film, or a viewing angle widening film for liquid crystal image display equipment.
2. Description of the Related Art
Cellulose acylate is a hygroscopic polymer, and a cellulose acylate film formed therefrom has in-plane retardation (Re) and retardation in the thickness direction (Rth) that vary accompanying changes in environmental humidity. When the cellulose acylate film is incorporated into a liquid crystal display device, if there are variations in the retardation, display unevenness occurs and the performance as a liquid crystal image display device is degraded. Re and Rth are defined in, for example, ‘Hatsumei Kyokai Koukai Giho’ (Journal of Technical Disclosure) (Technical Disclosure No. 2001-1745, published Mar. 15, 2001, Japan Institute of Invention and Innovation). It is therefore desirable for there to be little variation in Re and Rth accompanying changes in humidity; variation in Re and Rth accompanying a change in environment between 25° C./10% RH and 25° C./80% RH is preferably in the range of 0 nm to 90 nm, more preferably 0 nm to 60 nm, and particularly preferably 0 nm to 40 nm, and in this range the display unevenness can be eliminated.
Such a change in Re and Rth accompanying a change in humidity occurs in a short time (a few hours) with the change in humidity, is reversible, and is therefore different from moisture resistance (irreversible change caused by being exposed to high humidity for a long period of time (a few weeks or longer)).
Cellulose ester films are drawn so as to exhibit in-plane retardation (Re) and retardation in the thickness direction (Rth), and are used as retardation films for liquid crystal display devices, thereby widening the viewing angle. In recent years, a vertical alignment (VA) type liquid crystal display device has been developed, and there is a demand for a retardation film having higher Re and Rth. In order to meet such a demand for the retardation film, a technique employing a film formed by casting a solution of a cellulose acylate using a mixed ester of an acetyl group and a propionyl group has been disclosed (JP-A-2001-188128 (JP-A denotes a Japanese unexamined patent application publication)). However, even for the film described in this patent publication, the change in Re and Rth accompanying a change in humidity is not fully practical, and there is a desire for a technique that can improve this aspect.
There is a known method for forming a film by a so-called organic-inorganic hybrid technique employing a cellulose acylate and a reaction product of a hydrolyzable and polycondensable reactive metal compound (for example, JP-A-2003-171500, JP-A-2003-238688, and Journal of Applied Polymer Science, Vol. 58, 1263-1274 (1995)). However, the films formed by these known techniques have the problems that, since a cellulose acylate having a low total degree of substitution is employed, the change in Re and Rth accompanying a change in humidity cannot be reduced and it is difficult to make hydrolysis of the reactive metal compound proceed sufficiently, and there has been a desire for a technique that can solve these problems.
Furthermore, in order to carry out the above-mentioned known technique, water is often used for the hydrolysis of the reactive metal compound, and when a large amount of water is added in this process, the problem of film whitening sometimes occurs during film formation. The known organic-inorganic hybrid technique for formation of a film thus has various problems in practice, and there has been a desire for a technique that can solve these problems.
On the other hand, as an organic-inorganic hybrid technique for a cellulose derivative having a more highly cross-linked structure, there are a known cellulose derivative and an organic-inorganic hybrid film employing same, the cellulose derivative being formed by modifying cellulose with both an acetyl group and a substituent containing an ethoxysilyl group (for example, Journal of Applied Polymer Science, Vol. 58, 1263-1274 (1995)). However, since these known cellulose derivative organic-inorganic hybrid films employ hydrochloric acid, which is strongly acidic, in their preparation, when they are produced industrially, a film forming device can sometimes be corroded by hydrochloric acid that is generated during drying, or the cellulose derivative might be degraded by the acid, and there has been a desire for a technique that can solve these problems.